/*
day156_2
2020.1.31
*/
#include<iostream>
using namespace std;

class Solution {
public:
    int climbStairs(int n) 
	{
		if(n<=2) return n;
		
        int a1=1,a2=2,a3=3;
        int cur=3;
        while(cur<n)
        {
        	a1=a2;
        	a2=a3;
        	a3=a1+a2;
        	++cur;
		}
		return a3;
    }
};

int main()
{
	return 0;
}
